Detergent manufacturers have long recognised the need to control water hardness to ensure adequate cleaning by detergents. The detergency builders used in the past have been of two main types, namely sequestering builders and precipitating builders. A typical precipitating builder is an alkali metal carbonate, especially sodium carbonate. While from a cost point of view sodium carbonate would be an attractive builder, it has at least two significant disadvantages. Firstly, sodium carbonate alone is not usually capable of reducing the calcium ion conentration in calcium hard water to sufficiently low levels to achieve good detergency under practical washing conditions. This is because crystal growth is inhibited by materials, in particular condensed phosphates, which can arise from the soiled laundry load, or be present as contamination in the detergent composition. Secondly, the use of carbonate ions to precipitate the calcium hardness from the water can result in the deposition of calcium carbonate on the washed fabrics. It is known that the calcium carbonate precipitate is produced in such a crystal type and such a particle size that deposition on the fabrics is favoured. The presence of certain crystal growth poisons in the wash liquor can encourage this deposition. Typical such poisons are inorganic phosphates carried into the wash liquor from the soiled fabrics in cases where the fabrics have previously been washed in a composition containing tripolyphosphate.
It has previously been suggested that the calcium ion concentration can be reduced by including in the compositions substantial quantities of a high surface area insoluble material to act as a seed crystal and crystal growth poison adsorbent. Thus GB No. 1 437 950 relates to detergent compositions containing both an alkali metal carbonate and about 15% high surface area calcium carbonate, particularly calcite. However, while the use of calcite may reduce the calcium ion concentration in the wash liquor the compositions are more difficult to handle and may lead to increased inorganic deposition on the fabrics. Also, the use of large quantities of such calcite in a composition may detract from the cost savings achieved from using sodium carbonate.
The calcium ion concentration in a wash liquor can be reduced to sufficiently low levels by the use of, for example, a sequestering builder material such as sodium tripolyphosphate, and considerable commercial success has been achieved with phosphate-built formulations. However, it has now become apparent that, under some conditions, the discharge of significant quantities of phosphates into waste waters may produce environmental problems. There is therefore an increasing desire in some countries to reduce the level of phosphorus in detergent compositions.
It is known to provide detergent compositions in which at least one component is treated in such a manner that it becomes effective in the wash liquor only after a specific delay. Thus, for example, U.S. Pat. No. 4 040 988 (Procter & Gamble Company) discloses a detergent composition containing two specific granules. The first contains sodium carbonate and calcite. The second, which is treated in such a manner as to delay its dissolution in the wash liquor, contains a sequestering builder such as sodium tripolyphosphate, sodium silicate and a detergent active material. It is said that such a composition gives satisfactory depletion of calcium hardness from the water while utilising a lower total content of phosphorus than hitherto.
By delaying the dissolution of the sequestering builder, its effect as a calcium carbonate crystal growth poison is said to be reduced. We have discovered, however, that such compositions may not reduce the free calcium ion concentration to sufficiently low levels if the wash liquor already contains a crystal growth poison.